(a) Field of the Invention
The invention relates to a data transmitter for delivering data to a transmission line, and in particular, to a differential data transmitter having the advantages of energy saving, being able to precisely control the common-mode level, and being wide in operational frequency width.
(b) Description of the Prior Art
Data transmission lines and buses are used for transferring data between computer components and other digital data systems. Although generally the data processed by the computer components is in a single-ended form, i.e., “high” or “low”, differential transmission lines are generally employed to transmit data between the CPU and other computer components. The reason is that single-ended lines are subject to the influence of common mode noise while differential transmission lines are not. To state in detail, the data are denoted by the voltage differential between two lines in a differential transmission lines system, and the voltage differential remains the same when both lines are subjected to external influences.
If single-ended data is to be transmitted by means of differential transmission lines, a data transmitter is needed to convert the single-ended data to differential data, and at the receiving end, a data receiver is needed to convert the data from differential to single-ended form.
FIG. 6 shows a data transmitter disclosed in U.S. Pat. No. 5,694,060. As shown in FIG. 6, the transmitter includes a first and a second conduction paths connected in parallel between a node A and a node B; switches 61, 62 connected in series in the first conduction path, wherein the switch 61 is located near node A, while the switch 62 is located near node B; switches 63, 64 connected in series in the second conduction path, wherein the switch 63 is located near node A, while the switch 64 is located near node B; a fixed current source 65 for providing current to the first and second conduction paths via node A; and a fixed current source 66 for receiving current from the first and the second conduction paths via node B.
The switches 61, 62, 63, 64 respectively receive the input single-ended binary signal or its reverse direction signal such that when the switches 61 and 64 are turned on, the switches 62 and 63 are cut off, and turned on vice versa. The differential binary output signal is pulled out by node C and node D.
By comparing this data transmitter with the conventional one, although there is an improvement in energy saving, there are still some drawbacks as follows:
(1) Difficulty in Controlling Common-Mode Level
As the voltage drop of the fixed current source cannot be controlled, the voltage of node A and B cannot be determined either, thereby, the common-mode level is difficult to control.
(2)The externally connected resistance (100Ω) decides the width of the operational frequency of the data transmitter, and there is room to upgrade the operational frequency. As the fixed current sources 65, 66 possess very high output resistance, the width of the operational frequency of the data transmitter depends on the externally connected resistance (100Ω) of the transmission line. The width of operational frequency is inversely proportional to Req×C, wherein Req denotes the observed equivalent resistance from the transmission line to the data transmitter, and C denotes the capacitance of the transmission lines.
FIG. 7 shown a data transmitter disclosed in U.S. Pat. No. 5,519,728. As shown in FIG. 7, the data transmitter includes a first and a second conduction paths connected in parallel between node A and node B; switches 71, 72 connected in series in the first conduction path, wherein and the switch 71 is located near node A while the switch 72 is located near node B; switches 73, 74 connected in parallel in the second conduction path, wherein the switch 73 is located near node A while the switch 74 is located near node B; a fixed current source 75 for supplying current to the first and the second conduction paths via node A; and a resistor RB for receiving current from the first and the second conduction paths via node B, wherein the switches 71, 72, 73, 74 are respectively receiving the input single-ended binary signal or the reverse direction signal thereof such that when the switches 71 and 74 are turned on, the switches 72 and 73 are cut off; and vice versa. The differential binary output signal is pulled out by node C and node D.
By comparing this data transmitter with the one shown in FIG. 6, a resistor RB is employed between node B and ground point to replace a fixed current source. As a result, the voltage at node B is determined by the magnitude of the current of the fixed current source 75 and the resistance of the resistor RB determine. Therefore, the drawback of being difficult in controlling common-mode level of circuit of FIG. 6 is overcome. However, since the resistance of the resistor RB is normally much greater than the resistance (100Ω) of the externally connected transmission lines, the width of the operational frequency of the data transmitter is still determined by the resistance (100Ω) of the externally connected transmission lines.